Since filtering devices become clogged by the particles which they retain from the flows to be filtered, in particular plastics melt flows, and thus the passage of the fluid to be filtered would be disturbed in the long run, it is necessary to clean the filtering devices. Often filtering devices are used in which the passing medium is subdivided by the assistance of two guide channels into two partial flows, each partial flow is filtered separately, and subsequently the two partial flows are united again. In such filtering devices, the filtering elements therefore can be cleaned in that a control device, with injection molding devices and the like also termed backflush attachment, is provided by means of which it is possible to control the passage of the fluid to be filtered. In doing so, in the backflush position, the fluid enters a guide channel in a flow direction which is equal to the usual flow direction, flows through the associated filter and subsequently flows through the second filter and returns in the second guide channel, i.e. contrary to its flow direction in the normal state of operation. In doing so, the fluid flow entrains the particles adhering to the second filter. Thereafter, the fluid polluted with these particles is led out of the backflush attachment via control elements. Thus, the filtering device can be cleaned in a simple way. As a rule, the control elements of known backflush attachments include reductions or steps as compared to the remaining flow channels in the housing of the backflush attachment, which are necessary so as to maintain a certain pressure within the filtering device. Pollutants adhere to these reductions, or steps, respectively, which may block the control elements and thus greatly reduce the useful life of such backflush attachments. Moreover, the control elements of the backflush attachments frequently get into contact with the atmosphere resulting in a coking of the fluids, in particular in case of plastics melts. By this, hard layers are formed on the control elements which will have to be removed at regular intervals since otherwise the control elements may also become blocked thereby.
From DE 195 09 059 C1, a control device is known in which the supply to and discharge from two guide channels is controlled by means of rotary pistons. The flow channels in the rotary pistons have, however, a substantially smaller cross-section than the guide channels so as to provide for a pressure build-up, resulting in shoulders, or steps, respectively, at the channel transitions, primarily at the entry into the rotary piston, with deposits forming thereat which reduce the operability of the control device. In addition, the fluid is highly deflected in the rotary piston, and this deflection also contributes to the formation of deposits in the channels of the rotary piston in a detrimental manner. The pronounced deflections of the channels in the rotary piston do not only have the disadvantage of forming deposits, but are also complex to produce and thus involve high production costs.
It is an object of the present invention to prevent these deposits in the channels of the rotary piston, so that the useful life of such a backflush attachment is substantially increased and, moreover, the maintenance time and costs are reduced.
Moreover, it is another object of the invention to reduce the deflections of the fluid by providing an appropriate path of the channels in the rotary piston so as to avoid further deposits and to simplify the requirements of manufacture and thus lower the production costs.